1. Field of the Invention
This invention relates to the electrodeposition of composite coatings comprising a layer of electrodeposited metal having small particles of a non-metallic solid material uniformly dispersed throughout said layer.
2. Prior Art
The electrodeposition of a layer of metal on to the surface of a substrate metal has long been employed to enhance or modify such properties of the surface of the substrate as its corrosion resistance, wear resistance, coefficient of friction, appearance and the like. The surface properties of the substrate can be further modified by the electrodeposition of composite layers comprising an electrodeposited metal having discrete particles of a non-metallic material incorporated therein. For example, diamond particles have been incorporated in an electrodeposited metal layer to improve the abrasive or cutting properties of a grinding wheel, particles of such materials as silicon carbide and aluminum oxide have been employed to improve the wear resistance of the electrodeposited metal layer, and particles of such materials as graphite and molybdenum disulfide have been employed to reduce the coefficient of friction of the metal layer. The metal matrix of the composite layer may be any of the metals that are normally electrodeposited from aqueous electrolyte solutions and include such metals as copper, iron, nickel, cobalt, tin, zinc and the like.
The classic procedure for incorporating discrete particles of a non-metallic material in a layer of electrodeposited metal involves allowing the finely divided particles contained in the electrolyte solution to settle onto the generally horizontal surface of a substrate metal onto which surface a layer of a metal is simultaneously being electrodeposited. The layer of electrodeposited metal forms a metal matrix in which the nonmetallic particles are entrapped and thereby physically bonded to the surface of the substrate metal. This general procedure is exemplified by the process disclosed in U.S. Pat. No. 779,639 to Edson G. Case, and modifications thereof are disclosed in Pat. Nos. 3,061,525 to Alfred E. Grazen and 3,891,542 to Leonard G. Cordone et al. In order to promote the co-deposition of non-metallic particles in a electrodeposited metal matrix it has heretofore been proposed that a deposition promoter, usually a surface active agent, be applied to the surface of the finely divided particles of non-metallic material, or be added to the electrolyte solution in which the non-metallic particles are suspended, so that the particles suspended in the electrolyte solution will cling to the surface of the cathode when brought into contact therewith while the metal of the metal matrix is simultaneously being electrodeposited from the electrolyte solution onto the surface of the cathode. This general procedure is exemplified by the process disclosed in U.S. Pat. No. 3,844,910 to Alfred Lipp and Gunter Kratel.
In the Lipp et al process an amino-organosilicon compound, for example, gamma amino-propyl-triethoxy silane, is employed to promote the incorporation of non-metallic particles, for example, silicon carbide, in a layer of electrodeposited metal such as nickel. The amino-organosilicon compound can be added directly to the aqueous electrolyte solution or, preferably, it can be applied to the surface of the non-metallic particles before they are added to the electrolyte solution. In either case the presence of the amino-organosilicon compound in the electrolyte solution results in a substantial increase in the amount of non-metallic particles incorporated in the layer of electrodeposited metal over the amount that is incorporated therein when no such deposition promotor is present in the plating solution. Nonetheless, the Lipp et al process is subject to several operational limitations that limit the usefullness of the process, and the composite coated products of the process, for many purposes. Specifically, the total amount of non-metallic particles (that is, the total weight of the particles) that can be incorporated in the electrodeposited metal coating even under optimum conditions is less than the amount of these particles required for many applications, and in addition there is a practical limit on the size of the particles of non-metallic material that can be usefully employed in the process. That is to say, when the size of the non-metallic particles employed in the Lipp et al process exceeds about 10 microns the amount (that is, the weight) of the non-metallic particles incorporated in the layer of electrodeposited metal tends to decrease in rough proportion to the increase in the average size of the particles.
There is an important and heretofore unfilled need (for example, in the manufacture of grinding wheels) for composite coatings having a greater amount of larger size particles of the non-metallic material in the electrodeposited metal layer than can be produced by any of the prior art processes known to me. Accordingly, I have carried out an intensive investigation of the factors and the problems affecting the production of such coatings, and as a result of my investigation I have discovered that there is a substantial and surprising improvement in the amount and particle size of the non-metallic material in the composite coating when certain amphoteric surfactants are employed as deposition promoters in the process. Specifically, I have found that when certain substituted imidozolinium compounds are employed as deposition promoters in the process, it is possible to incorporate particles of non-metallic material of up to 150 microns or larger in size in the electrodeposited metal matrix without a concommittant decrease in the amount or weight of the particles incorporated therein.